It is code required in Canada. I personally wouldn't install a tub without one, especially if you have kids. The reason for the overflow is in case you fill the tub up to take a bath and you somehow forget about it, no overflow = water coming through ceiling from below
there are no (new) mobile homes with overflows. The only time you see them is if it's been remodeled. I guess the mobile home plumbers can't figure out the waste/overflow kits so they just omit them and cover the overflow hole instead.